Common reasons health insurance claims are denied or underpaid
- Service coded as 'not medically necessary' — often reversed with a doctor's letter
- Out-of-network provider treated as in-network emergency (balance billing dispute)
- Pre-authorization wasn't obtained — even when the ER didn't allow time to get it
- Incorrect billing codes submitted by the provider — not your fault but your problem
- Coordination of benefits errors when you have more than one insurance plan
- Annual or lifetime benefit limits applied incorrectly (ACA prohibits most lifetime limits)
- Mental health parity violations — insurer applied stricter limits than medical benefits
Three real denial patterns we catch
Case 1 — Retroactive medical-necessity denial after prior-auth approval (Florida, employer ERISA plan)
What happened. A 47-year-old enrollee in a self-funded employer plan administered by a large national TPA had a laparoscopic cholecystectomy at an in-network ambulatory surgery center. The plan issued a written prior-authorization approval six weeks earlier with a confirmation number and a stated approval window. Eight weeks post-op, the EOB came back paying $0 of $22,400 in billed charges.
The underpayment. EOB CARC 50 (“non-covered services because this is not deemed a medical necessity”) plus RARC N115 referencing a local coverage policy. Allowed amount listed as $11,800; plan paid $0; patient responsibility shown as $11,800 after in-network discount. Member also received a $10,600 balance bill from the surgeon.
How we caught it. The prior-auth approval letter (PDF, dated, with confirmation number) referenced CPT 47562 and ICD-10 K80.20 — the exact codes on the submitted claim. The denial letter cited a different rationale (“conservative management not documented”) than the EOB's CARC 50, and never mentioned the prior auth at all. Two-document mismatch is the tell.
Why it happens. Retroactive medical-necessity reversals after a written PA are formally barred by most plan documents but quietly common when post-payment audits flag a case. The plan bets that 60-70% of patients won't escalate past the internal stage, and that providers will write off rather than fight.
The counter. ERISA §503 and 29 C.F.R. §2560.503-1(g) require the adverse benefit determination to state the specific reason and the plan provision relied on — a prior-auth approval is itself a plan determination the issuer is generally estopped from reversing absent fraud or material change. Internal appeal under 29 C.F.R. §2560.503-1(h) within 180 days, then external review under ACA §2719 (42 U.S.C. §300gg-19). Dollar swing here: $11,800 plan-paid plus extinguishing the $10,600 balance bill = ~$22,400.
Takeaway: a written prior-auth with a confirmation number is the single strongest document in a medical-necessity dispute — never let it sit in a drawer.
Case 2 — Out-of-network ER anesthesiologist at an in-network hospital (Texas, ACA marketplace plan)
What happened. A 34-year-old on a Silver marketplace plan went to an in-network hospital ER with appendicitis, was admitted, and had emergency surgery the same night. Hospital and surgeon were in-network. The anesthesiologist and the assistant surgeon, both staffing the in-network facility, were out-of-network with the plan.
The underpayment. Total billed $38,900. Hospital and surgeon adjudicated normally at in-network rates. Anesthesia bill $6,400 and assistant-surgeon bill $3,200 processed at OON benefit level: plan paid 50% of an internal “UCR” figure of $2,100, leaving the patient with a $7,650 balance bill across the two providers, on top of the in-network deductible.
How we caught it. The facility was confirmed in-network on the plan's own provider directory the day of admission. Both ancillary providers billed POS 23 (emergency room) and POS 21 (inpatient hospital). The EOB applied OON cost-sharing to provider TINs that staffed an in-network facility — a textbook No Surprises Act trigger. The plan's own initial-payment notice did not include the NSA-required disclosure of IDR rights.
Why it happens. Plans still process facility-based ancillary claims through legacy OON logic because the IDR escrow, batching, and qualifying-payment-amount workflows live outside the standard claim engine. The patient cost-share gets calculated correctly only if someone manually flags the NSA path.
The counter. 42 U.S.C. §300gg-111 and the implementing rules at 45 C.F.R. §149.110-.140 cap the patient's cost-share at the in-network amount for emergency services and for ancillary services at in-network facilities, and prohibit balance billing. The provider-plan payment fight goes to federal IDR under 45 C.F.R. §149.510, not the patient's appeal. Dollar swing: ~$7,650 patient liability extinguished; cost-share recalculated to in-network deductible/coinsurance only.
Takeaway: if the facility was in-network and you didn't pick the anesthesiologist, NSA almost certainly caps your bill — the dispute is procedural, not a medical-necessity appeal.
Case 3 — Mental-health residential treatment cut short under a non-quantitative limit (California, employer ERISA plan with national behavioral-health vendor)
What happened. A 19-year-old with severe anorexia nervosa and a recent medical hospitalization was admitted to an in-network residential treatment center at $1,950/day. Treating clinicians documented BMI in the medically unstable range and recommended 60 days. The plan's behavioral-health vendor authorized 14 days, then issued a concurrent-review denial for “no longer meeting acute residential criteria.”
The underpayment. Days 1-14 paid at $1,950/day = $27,300. Days 15-46 (32 days) denied as not medically necessary, exposing the family to $62,400 in continued-care charges. EOB CARC 50 with vendor-proprietary criteria cited as the basis.
How we caught it. The plan's medical/surgical inpatient and skilled-nursing benefits used InterQual or MCG criteria with concurrent review every 5-7 days on a clinical-deterioration standard. The behavioral-health vendor used a proprietary acute-criteria checklist with concurrent review every 3 days and required active medical instability for continued residential authorization — a stricter standard than any med/surg analog. That asymmetry, on its face, is a non-quantitative treatment limitation (NQTL) violation.
Why it happens. Behavioral-health carve-outs run on separate utilization-review playbooks built before MHPAEA enforcement matured. The criteria, review cadence, and reviewer-credentialing standards drift more restrictive than the medical side, and most members never request the comparative analysis that exposes it.
The counter. MHPAEA (29 U.S.C. §1185a; 26 U.S.C. §9812; 42 U.S.C. §300gg-26) and the 2024 final NQTL rule require parity in processes, strategies, evidentiary standards, and other factors used to apply any treatment limitation. The CAA-2021 amendment requires plans to produce a written comparative analysis on request — failure to provide it is itself a violation. Internal appeal plus a written request for the NQTL comparative analysis; if not produced or non-compliant, escalate to external review under ACA §2719 and to DOL EBSA. Dollar swing: $62,400 in residential days plus eligibility for step-down PHP/IOP at parity terms.
Takeaway: in a behavioral-health denial, the most powerful question is not “was it medically necessary” but “show me the same criteria you apply to a medical/surgical inpatient stay” — and the plan must produce it in writing.
How to read your Explanation of Benefits (EOB)
Your EOB is not a bill — it's the insurer's accounting of what they paid and why. Key fields to check:
- Billed amount — what the provider charged
- Allowed amount — what the insurer says the service is worth (often much less)
- Plan paid — what the insurer actually paid after your deductible/copay
- Your responsibility — what you allegedly owe
- Reason codes — cryptic codes that explain why a claim was denied or reduced
If the reason codes don't match your actual situation — or if the allowed amount is far below what comparable providers charge — you have grounds for an appeal.
The health insurance appeal process (internal and external)
Step 1: Internal appeal
You have the right to an internal appeal within 180 days of receiving the denial. Submit a written appeal letter, your EOB, and supporting documentation (doctor's letter, medical records). The insurer must respond within 30 days for prior-auth disputes or 60 days for post-service claims.
Step 2: Expedited appeal (for urgent care)
If your health is at risk, you can request an expedited appeal. The insurer must respond within 72 hours.
Step 3: External review
If the internal appeal fails, you can request an independent external review. Under the ACA, insurers must abide by the external reviewer's decision. Success rates are significant — approximately 40% of external reviews overturn the insurer's decision.
Step 4: State insurance commissioner complaint
File a complaint with your state's Department of Insurance, especially for bad-faith denials, mental health parity violations, or emergency care disputes.